Gas sensors are known which are installed in an exhaust system of an internal combustion engine such as an automotive engine to detect a specified gas such as an oxygen concentration in exhaust gas (i.e., measurement gas). For instance, Japanese Patent First Publication No. 2007-199005 discloses a gas sensor which includes a sensor device which measures the concentration of a given gas component in the measurement gas, a device-side insulator porcelain which retains the sensor device therein, an atmospheric side insulator porcelain which covers a base end side of the sensor device to form an air atmosphere, a housing which retains the insulator porcelains therein, and an atmospheric side cover secured to a base end side of the housing. The atmospheric side cover includes a large-diameter portion on a front end side, a small-diameter portion on a base end side, and a shoulder between the large-diameter portion and the small-diameter portion. The shoulder presses a base end surface of the atmospheric side insulator porcelain through a biasing member to a front end side to place the atmospheric side insulator porcelain in contact with the device-side insulator porcelain. A seal made of a compressed sealing member such as talc is disposed between the housing and the device-side insulator porcelain to isolate between an air atmosphere formed as a reference gas inside the atmospheric side insulator porcelain and a measurement gas atmosphere in which the measurement gas exists.